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A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

I was asked by a magazineto review Alan Boss's new book The Crowded Universe. They asked for a review that was a much an essay on the field as a review of the book itself, which made it a very fun exercise. The following is based on the review that I got to write.

Two hundred fifty years ago, Immanuel Kant, in his Universal Natural History and Theory of the Heavens, laid out a remarkably modern-sounding account of the state of the universe. Moons go around planets. Planets go around stars. Stars go around the Milky Way. The Milky Way and other galaxies (“other Milky Ways,” he called them) go around something even larger. The solar system had an understandable origin, and inevitable consequences:

The planetary structure in which the sun at the centre makes the spheres found in its system orbit in eternal circles by means of its powerful force of attraction is entirely developed, as we have seen, from the originally distributed basic stuff of all planetary material. All the fixed stars which the eye discovers in the high recesses of the heavens and which appear to display a kind of extravagance are suns and central points of similar systems.

To paraphrase: gravity takes stuff and turns it into stars surrounded by planets, and it has done so everywhere you see a star in the sky.

For the first 240 years after the publication of Kant’s assertion, this fact could only be verified for only a single star in the sky: the sun. In 1995 Michel Mayor and Didier Queloz announced the discovery of the first planet orbiting a star other than the sun. Now, fourteen years later, almost 300 stars are known to have planets around them. It is not quite “all of the fixed stars which the eye discovers,” but it’s getting close. Kant was substantially correct. It had been accepted since the 17th century that our sun is not special, but is, instead, but one of many stars in the universe. Now, at the beginning of the 21st century, it is clear that our planets aren’t special either.

Except that some of our planets are still special.

It is tempting to describe the many planetary systems that have been discovered in the past decade and a half as simply weird. Rather than the orderly arrangement of planets that we have here in the solar system, with small planets close, large planets far, and everything going around the sun in satisfyingly circular orbits in a common disk (each one of these properties is “inevitable”, according to Kant, and according to most astronomers up until late 1995), we have instead found planets the size of Jupiter that orbit their stars closer than Mercury, planets with orbits as elliptical as some of the comets in the solar system, and planets with separations from their central star far beyond even the most distant objects detected in our solar system. Weird, indeed. The only type of planetary system that we haven’t found, it seems, is one like our own. Nowhere out there has there been anything quite like the solar system; nowhere out there is another Earth.

But even this special position that our home planet holds is now in jeopardy.

Alan Boss’s new book The Crowded Universe tells the story of the development and launch of NASA’s Kepler spacecraft, which was recently launched from the earth to go into orbit around the sun. Kepler’s 3 1/2 year mission is simple to state: find the Earths. Kepler, along with a similar ESO mission CoRoT, will be the first to finally have a chance to tell us whether planets like the one on which we live are as common as Kant would hope or as rare as some astronomers think.

Boss weaves the story of Kepler (surely a must-read cautionary tale for anyone contemplating a life in NASA mission development) with the larger story of the entire, now booming, field of exo-planets. As someone whose astronomical career has spanned the period Boss discusses, I’m glad someone was taking notes. It is fun to be able to go back to those days when each new planetary discovery was an exciting event with multiple teams struggling to outdo the others with firsts. First planet at the distance of the earth! First transiting planet! First multiple planet system! With the current richness of the exo-planet field it is easy to forget that almost all of this is under a decade old.

Boss gives the insider story not only of the Kepler mission development and the birth and childhood of the entire exo-planet field, but, in a stroke of luck for us all, he got to play a intimate role in the definition of planets in our own solar system, and he gives what I believe is the first account of some of the inner workings of the International Astronomical Union committee that first started trying to figure out what to do with Pluto and Eris and the things that we now call dwarf planets. The demotion of Pluto was unassailably reasonable, but the events leading up to this eventual demotion were some of the more publicly comical occurrences in recent astronomical history. Reliving these moments is an excellent reminder that for all of their command of the physics of the universe around them, astronomers, being human, have the capacity for nearly infinite folly.

But for Boss and The Crowded Universe, Pluto is just a distraction, and rightly so. The meat of his book is the race for finding something like the Earth. Sitting in the middle of the events, it would be easy to get caught up in the day-to-day (or perhaps committee meeting to committee meeting) details. But Boss, while detailing the daily work of himself and other scientists involved in the field, never ceases to forget that we’re privileged to live in such at a time when a nearly-Copernican-magnitude revolution is unfolding.

Yet even if Kepler and CoRoT find an abundance of planets, the 250 year old Kantian revolution will not be complete. The planets that these spacecraft might find could be the precise size of the Earth and could orbit their stars at the exact distance of the Earth, but while an astronomer might be willing to call such a thing Earth-like, most people will still want to know more. Does it have liquid water? Does it have a recognizable atmosphere? And, inevitably, the only thing that really matters, could it – no: does it – support life?

The answer to these questions will take decades or more to answer. Kepler and CoRot are simply first steps along the way. In the meantime, we can perhaps take solace from Kant:

I am of the opinion that it is not particularly necessary to assert that all planets must be inhabited. However, at the same time it would be absurd to deny this claim with respect to all or even to most of them.

It took 240 years to prove him mostly right the first time. With a little bit of luck and a little bit of perseverance and, as Boss shows, a lot of the day-to-day work of astronomers around the world, the final step might come just a little bit faster.

What interests me is what caused our system to be so precise and ordered, and what made all these other systems so weird. Why are our major planets on circular orbits, but all these exoplanets on ellipticals? Perhaps it all comes down to the anthropic principle - if it wasn't this way, life wouldn't have evolved, and we wouldn't be here to discuss it anyway.

But keep in mind that we are currently "very biased" since it is much easier to spot gas giants orbiting close-in (rapidly) on highly elliptical orbits. I suspect that once our processes get more refined that we will find gas giants orbiting 1AU to 10AU from the host star on roughly circular orbits. And when you have gas giants on roughly circular orbits, you can have habitable terrestrial planets. -- Kevin Heider

Nonsense Anonymous, what has metaphysics got to do with hunting for exoplanets anyway? Modern physicists are perfectly correct in rejecting any kind of "holistic logic" -- surely a contradiction in terms anyway.

I like the great new artwork in the header. And, as usual, this is a great article.

I got into a bit of trouble a few years ago discussing Kant. We wound up arguing over a word. It wasn't a complicated word like "metaphysicist" or "pseudophysicist", it was the ordinary, everyday word, "object". To him, an object was a line of code in C++ programming language which represents "something". Kant did not have a computer. But my debator persisted in saying an object is a line of code in C++ programming language. :?

I think he was trying to make the point that words mean different things to different people. In "the context in which I most use the word" it would be most like Mike Brown uses the word. But since the Kant discussion I have met and frequently conversed with Tony Dunn, who is a computer person, and in those contexts an "object" is more similar to my debator's. A more general definition is that an object is something in the material world. Yes, there are imaginary objects, too.

The stability of the Solar system was a hot topic of study long before other solar systems were known. Laplace thought he "proved" the Solar system was stable, but later some graduate student found a flaw in the proof which invalidated the whole thing. Still later somebody proved that the n-body problem cannot be solved if n > 3.

But there's relative stability and fortunately for us, our own Solar system has proven to be relatively stable compared to other systems. This may bode ill for the current definition of a "planet" since it's based on orbital dominance. The key appears to be that Jupiter and Saturn are too small compared to the mass of the Sun to have much influence over each other. Though we're not out of observational bias yet, it appears so far that in most systems, the planets are larger and can indeed affect each other's orbits. In fact, it's believed that Jupiter and Saturn entered into a period of chaotic interaction 3 1/2 billion years ago, which cleared out most of the asteroid belt.

It was a 2:1 resonance. I designed an experiment on GravitySimulator to test that and it worked, Jupiter and Saturn can throw each other around. This shows up on the standard simulation of the Solar system that comes with the program, though it's kind of subtle it is plainly visible if you look.

About observational bias: the chance of seeing a planet using the Kepler probe's technique is the diameter of the star divided by the diameter of the planet's orbit. A randomly located alien astronomer using the Kepler probe's technique has a .0047 chance of seeing Earth, a little less than half a percent. Since Jupiter's orbit is about five times Earth's the chance that he will see Jupiter is five times less!, and even less for Saturn. He ("it") will have a better chance of seeing Venus and if he can see Venus, then because the plane of the orbits is similar he has a 12% chance of seeing Earth.

About that sampling of 100,000 stars; it's actually a sampling of 471 stars for "Earthoids" or "terroids" (I actually like the second name better after thinking a little about it, it rolls off the toungue easier). So far it seems that about 10% of stars have any planets so it goes down to 47.1 terroids in the sample. I fear, because of the instability, that only a small percentage of stars with any planets have terroids so the sample goes down again to 4.71 (if 10% of stars with planets have terroids). We are getting very near the point of not finding any through sheer bad luck. I think I'll take a few venusoids just to keep Star Trek alive...

Anyway I have optimistically chosen square #13 in the office Kepler betting pool (13 terroids). Anybody else care to make a guess?

Gravity Simulator:"This shows up on the standard simulation of the Solar system that comes with the program"

This is true. But Jupiter moves Saturn more than Saturn moves Jupiter. Saturn then moves Uranus. Since Uranus spends a significant amount of time inclined more than 1 degree to the invariable plane of the solar system, Uranus and Saturn seems to have the most obvious "wobble" to their orbits. But they are dominant planets. A "mass-poor" centaur between the orbits of Saturn and Uranus does not stand a chance "long-term".-- Kevin Heider

I think if Jupiter was even a little more massive compared to the Sun that it might be much more effective at tossing these other planets around. What the observations are showing me so far is that there seems to be some fundamental mass, I don't know, maybe two Jupiters? beyond which a solar system is not stable over the timeframe defined by the age of our own Solar system. I think if Saturn were the size of Jupiter then probably Jupiter would wind up a "roaster".

But that's speculation, Kepler is designed to answer these questions.

Anything inside Neptune is in trouble. Outside there are stable resonances like 3:2 or 5:2.

Thank you so much for bringing these things up, Kevin. I have a question though, how do you make links on this board? That's information we probably all could use...wish we could post images...

First, I changed the masses of the gas giants to 10 Jupiter masses. The Solar system fell apart almost immediately, well, I was expecting that. Saturn fell out first and kicked Jupiter into an eccentric orbit. In 3200 years we had lost Mars. By this time Uranus was in a highly eccentric orbit. When I decided to cut it off Uranus had receded to more than 30 billion kilometers, Saturn to 200 billion.

Well, that was kind of extreme (humans like to see spectacular results immediately). So I started over and just made Saturn the mass of Jupiter. That was more subtle. The orbit of Uranus changed pretty significantly but after 30 or 40 thousand years it seemed to settle in a bit. Do remember that 30 or 40 thousand years is not a long simulation.

I decided that I wasn't doing this very systematically so I started over again, this time making all the gas giants one Jupiter mass. That simulation has now run 100,000 years, still a very short simulation, but there have been some pretty spectacular results considering the short time frame. So I'm going to publish the results and wait for suggestions about how I can make this more systematic and informative.

As far as system stability is concerned, I don't think planetary mass per se is enough: we need to think about how far apart they are initially. That suggests an important factor would be the initial surface density of the protoplanetary disc - denser discs presumably lead to a greater probability that the resulting planets will throw each other into funny orbits or out of the system. Also, there's planetary migration to consider: if many of the planets fall into the star, they might end up leaving something stable behind.

:how do you make links on this board?I use basic HTML code, ie: you need to enclose the following in "< a" and ">"h ref="http://www.webpage.com">generic text< /a(No space between h and ref, ie: href and no space between < and /a)See Hypertext on Wikipedia for more.

:if Jupiter was even a little more massiveYes, if Jupiter was more massive it is a given that it would throw more things around, esp. things that are closest to it (asteriod belt, Mars, Saturn, comets)

:this time making all the gas giants one Jupiter mass:Just that little increase in mass has wrecked the Solar systemUmmm. I'm not sure I would call that a little change... :-) That would be an entirely different planetary system that would have had a completely different evolutionary history.

:The inner planets don't seem to have been affected muchI am guessing that is because the Sun has a stronger hold on them.

there seems to be some fundamental mass, I don't know, maybe two Jupiters? beyond which a solar system is not stable over the timeframe defined by the age of our own Solar system.That probably has a lot of truth to it. Just as not all animals age at the same rate (dogs/humans), I don't expect planetary systems to age (evolve) at the same rate. Besides massive stars don't even live that long.

:how do you make links on this board?I use basic HTML code, ie: you need to enclose the following in "< a" and ">"h ref="http://www.webpage.com">generic text< /a(No space between h and ref, ie: href and no space between < and /a)See Hypertext on Wikipedia for more.

:if Jupiter was even a little more massiveYes, if Jupiter was more massive it is a given that it would throw more things around, esp. things that are closest to it (asteriod belt, Mars, Saturn, comets)

:this time making all the gas giants one Jupiter mass:Just that little increase in mass has wrecked the Solar systemUmmm. I'm not sure I would call that a little change... :-) That would be an entirely different planetary system that would have had a completely different evolutionary history.

:The inner planets don't seem to have been affected muchI am guessing that is because the Sun has a stronger hold on them.

:there seems to be some fundamental mass, I don't know, maybe two Jupiters? beyond which a solar system is not stable over the timeframe defined by the age of our own Solar system.That probably has a lot of truth to it. Just as not all animals age at the same rate (dogs/humans), I don't expect planetary systems to age (evolve) at the same rate. Besides massive stars don't even live that long.

One of the interesting peculiarities of the history of extrasolar planets discoveries is that the first confirmed extrasolar planetary system is so rarely acknowledged. Terrestrial-mass extrasolar planets have been known since 1992 but this never seems to get mentioned in the news stories about super-Earths which proclaim the discoveries to be the "smallest known extrasolar planet" or whatever. Record to beat is 2 lunar masses, set in 1994...

Thank you for the important suggestion about distance. This implies a systematic series of simulations which might get some answers pretty quick.

I'm still answering Kevin's implied questions about small changes. Therefore I haven't started on the distance simulations. I get the idea from all this that maybe I ought to do the least stable simulations first because I can get a bunch of important answers before the heat death of the Universe sets in.

I've made some errors, of course one of those was the science-fictional/yahoo rush to blow up the solar system. I could have gotten results even quicker.

One error that I shouldn't have made because I had experience with it was failure to erase Mercury, Venus, and Mars. The machine has to calculate those objects' position and it takes time. They don't really affect the main problem of the giant planets.

These are artificial solar systems. So the design of them should be to answer questions rather than to try to guess at what a natural solar system might look like.

Hi, Kevin :)

Thanks for your excellent advice and great links. I have now run the 1J simulation for over a million years, which is still too short (oh, well...). Uranus has gone to 3.03 to 2.58 billion kilometers and seems the least stable. I have seen chaotic excursions from all four planets but Jupiter's seems the least extreme after the initial perturbation and Uranus seems the most extreme.

Indeed, I think we look outward and then define "normality" as "weird," when in fact the more evidence we cull, we ought to think the opposite. The anthropic principle indeed! (Thanks to Richard Dawkins for enlightening me on it!).

I hear you about comment capacity. But I'm really interested in Kevin's results, so I have started a blog to absorb maybe some of the additional comments being generated by the very interesting topics you bring up: http://mikeemmert.blogspot.com/Of course, you, Tony Dunn, Kevin Heidar, Vagueofgodalming, Laurel Kornfeld (whom I imagine is cheering at the resuls of my orbital domination simulations) or anybody else is welcome to post. I can't contact Kevin that way, he "...hasn't posted a blog yet".

Mike --No no no, my response to Kevin was a lame attempt at humor. Reread his post. He says Charon will become a >comment<. I have made that very type myself. And I can't type plant without planet coming out. To my knowledge there is no practical limit to comments here. But a comet or two would really jam things up.Mike

Well, it looks like I stuck my foot in my mouth again. Actually I've needed to start my own blog for some time, now I've gone and done it, glad I did, too.

You can read details of my simulations on my blog. I want to see details of the Haumea eclipse series and you have a link to that.

I think the results are relevant here, though. A preliminary result is that what seems to matter most to stability is the total mass of the planets. By good fortune I did 15 J's arranged in different configurations on my first three "baseline" simulations and they lasted between 50 and 100 thousand years. That falls within the range that you can pretty much totally trust something like GravitySimulator.

So, 15 Jupiter masses is the "critical mass" for a solar system with the same spacing as the best-known example. I haven't gotten to Vagueofgodalming's question yet because I was busy establishing base simulations. Not only did I make typos in those simulations, I also made "brain-o's"! First thing you figure out when doing simulations is that you don't know what you're doing, you have to have your hands on it to know that.

Kevin cut his Chiron simulation at 100 ky, as he explains, these type programs become inaccurate if you run them for too long. I don't know how he defines accuracy, however. I think for hypothetical or imaginary objects you can go considerably longer, since the results will be similar to a real system.

The Chiron simulation works here because it demonstrates how "roasters" form, and they are the most common type of exoplanet discovered so far. But there is another post coming up here, as you announced, "Life and Death in the Kuiper belt". And I think Chiron might be more on-topic there. I don't know; I haven't read the post yet!

My disclaimer was basically about centaurs since they are at the mercy of the dominant planets, unpredictable outgassing, possible close passes to other unknown small bodies, possible splitting when passing too close to a gas giant, uncalculated perturbation by a large moon of a gas giant, etc. Just one unaccounted perturbation for a centaur will throw off the accuracy of the longer term predictions.

Simulations of our stable planets are reliable for millions of years, because of the well balanced exchane of energies that has been established over the last 4.5 billion years.-- Kevin Heider

Well, I've done the simulations to try to give some kind of answer about Vagueofgodalming's question. Short answer: yes, indeed, spreading the distance between the planets out does make the system last longer before crashing. Details on my blog, http://mikeemmert.blogspot.com/ :D

Thanks for your answer, Kevin. All these things you mentioned do indeed affect real systems. And then there are rounding errors that build up when you are in effect adding millions of numbers together.

There can be other errors, too. For instance, I was fortunate enough in one of these recent simulations to see the encounter that threw the planets out of kilter. Both were ejected from the Solar system. That's obviously impossible, goes against conservation of energy, so what happened?

Tony Dunn explained it this way: in (for instance) his Solar system simulation, the machine is given the positions and velocities of all the objects on the board. It then calculates the total gravitational influence of all the other objects to come up with their positions and velocities 65536 seconds later. In the vast distances of the Solar system, the percentage of the distance moved in that time compared to the distances in the Solar system is very tiny, so the error is negligible.

However, if you have two giant planets, 3.75 Jupiter masses, coming together, their velocity increases enormously. Thus the percentage of the distance traveled in 65536 seconds compared to the total distance between them is large. In fact, in that amount of time they can actually fly right through each other!

The time step is adjustable, though. When simulating a flyby of two Triton-sized objects past Neptune I stopped that down to as little as an eigth of a second and that gave an accurate answer. However, at that time step the trip from Neptune's Hill sphere to the altitude of Triton's orbit took too long (days) so the simulations were started with a 256 second time-step and I had to micromanage the time step. Not a problem on that particular simulation, but I couldn't do that in these chaos simulations because you never know when they're going to collapse.

Yes, rounding errors do add up. But you can always use more bits (ie: 18-bit instead of 14-bit). I was careful with Chiron because that was just one clone simulated to show how Chiron could migrate to the iner solar system. But by using 50 clones you can make more reliable "short term predictions". For example, I can say with high confidence that on 5290-07-07 centaur 5145 Pholus will pass ~100Gm from Neptune. This is because Pholus will not make close passes to other gas giants before then and all 50 of my clones make the same pass. But each one of my clones will be perturbed slightly differently, and with longer simulations (with other close passes to the gas giants) the clones (and errors) will start to deviate more and more.-- Kevin Heider

With a crowded universe, what are the odds of the exact same human being being created in this universe (or future universes)again? If it happened once, it can happen again, and since everything runs on almost infinity out there, the number should be close to 100%....